1. Field
The disclosed concept pertains generally to electrical switching apparatus and, more particularly, to bidirectional direct current electrical switching apparatus, such as, for example, bidirectional direct current relays and circuit breakers.
2. Background Information
Plural parallel strings of series-connected direct current (DC) electrical generating modules (e.g., without limitation, photovoltaic (PV) or solar generating modules) can employ a fuse located in the positive conductor of each string. The fuse only protects against a reverse over current when the corresponding string shorts and is back fed by the other PV strings which are bussed together at a main DC bus in a combiner box.
It is known to employ fuses for over current protection in combination with diodes to block reverse current.
It is believed that fuses are essentially useless in PV power systems since such fuses are sized at 125% and typically must open at 150% of full load current, while the maximum short circuit current for PV arrays does not significantly exceed 100% of full load current. In PV power systems, fuses protect conductors, such as wires, from over currents. Over currents can only result from a back feed condition. Formed feed currents for silicon PV modules are typically limited to about 7.5 A by the structure of the PV modules, which cannot produce more than about 7.5 A even when short circuited. Therefore, string fuses are useless for providing protection from a forward feed fault, or bus faults that occur above the string fuses.
In a PV generating system, the parallel strings of series-connected DC electrical generating modules supply DC power to the main DC bus in the combiner box. A substantial reverse over current can be caused when one of the strings shorts and is back fed by the other PV strings which are bussed together at the main DC bus. The reverse over current is the greatest when the short occurs just prior to the last DC electrical generating module in a string.
There is room for improvement in bidirectional electrical switching apparatus.